Chemical Composition of Cement
The raw materials used for the manufacture of cement consist mainly of lime, silica, alumina
and iron oxide. These oxides interact with one another in the kiln at high temperature to form
more complex compounds. The relative proportions of these oxide compositions are responsible
for influencing the various properties of cement; in addition to rate of cooling and fineness of
grinding. Table 1.4 shows the approximate oxide composition limits of ordinary Portland
cement.
Table 1.4. Approximate Oxide Composition Limits of Ordinary Portland Cement
Oxide
Per cent content
CaO
60–67
SiO2
17–25
Al2O3
3.0–8.0
Fe2O3
0.5–6.0
MgO
0.1–4.0
Alkalies ( K2O, Na2O)
0.4–1.3
SO3
1.3–3.0
The identification of the major compounds of cement is largely based on Bogue’s equations
and hence it is called “Bogue’s Compounds”. The four compounds usually regarded as major
compounds are listed in table 1.5.
Table 1.5. Major compounds of cement
Name of Compound
Formula
Abbreviated Formula
Tricalcium silicate
3 CaO.SiO2
C3S
Dicalcium silicate
2 CaO.SiO2
C2S
Tricalcium aluminate
3 CaO.Al2O3
C3A
Tetracalcium aluminoferrite
4 CaO.Al2O3.Fe2O3
C4AF
It is to be noted that for simplicity’s sake abbreviated notations are used. C stands for CaO, S
stands for SiO2, A for Al2O3, F for Fe2O3 and H for H2O. The equations suggested by Bogue for
calculating the percentages of major compounds are given below.
C3S = 4.07 (CaO) – 7.60 (SiO2) – 6.72 (Al2O3) – 1.43 (Fe2O3) – 2.85 (SO3)
C2S = 2.87 (SiO2) – 0.754 (3CaO.SiO2)
C3A = 2.65 (Al2O3) – 1.69 (Fe2O3)
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C4AF= 3.04 (Fe2O3)
In addition to the four major compounds, there are many minor compounds formed in the kiln.
Two of the minor oxides namely K2O and Na2O referred to as alkalis in cement are of some
importance and Expressed in terms of Na2O. These alkalis basically react with active silica in
aggregate and produce what is called alkali-silica gel of unlimited swelling type under
favourable conditions of moisture and temperature in voids and cracks and further it causes
disruption and pattern cracking.
Tricalcium silicate and dicalcium silicate are the most important compounds responsible for
strength. Together they constitute 70 to 80 per cent of cement. The average C3S content in
modern cement is about 45 per cent and that of C2S is about 25 per cent.
The calculated quantity of the compounds in cement varies greatly even for a relatively small
change in the oxide composition of the raw materials. To manufacture a cement of stipulated
compound composition, it becomes absolutely necessary to closely control the oxide
composition of the raw materials.
SO3 also appear in cement analysis which comes from adding gypsum (4-6) % during clinker
gridding. The Iraqi and British specification for normal high rapid Portland cement pointed that
SO3 content must be between ( 3-2.5 )% according to type of cement and C3A content.
The percentage of MgO in cement which is come from Magnesia compounds in raw material.
is about ( 4-1)% and 5% as maximum range to control expansion from hydration of this oxide
in hard concrete.
An increase in lime CaO content beyond a certain value makes it difficult to combine with
other compounds and free lime will exist in the clinker which causes unsoundness in cement.
Insoluble residue is that part of the Cement non-soluble in hydrochloric acid HCl and arise
mainly from non active silica to form cement compounds dissolved in this acid therefore it
express the completeness of the chemical reactions inside the rotary kiln.
Loss on Ignition
Loss on ignition is calculated by heating up a cement sample to 900 – 1000°C until a constant
weight is obtained. The weight loss of the sample due to heating is then determined. A high loss
on ignition can indicate prehydration and carbonation, which may be caused by improper and
prolonged storage. Iraqi specification limits is 4% as maximum for normal and rapid hardening
Portland cement.
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Setting and Hardening of Cement
Setting Process: The process of losing plasticity with time and becoming dense, which is
formed by mixing the cement and water. It happened in two stage initial setting and final
setting.
Hardening Process: The process of forming hardening cement paste, which loses plasticity and
its strength increases with time.
Factors such as fineness and types of cement, blend of gypsum, age, water content, temperature
and humidity which influence setting and hardening of Portland cement.
Setting is important in concrete work to keep fresh concrete plastic for enough time which helps
the processes of transporting, casting, and compaction.
There are four stage occurs during setting of cement, First : when mixing cement with water, a
rapid heat evolution, lasting a few minutes, This heat evolution is probably due to the reaction
of cement compounds then, heat evolution ceases quickly. Second: this stage called “dormant
period” and last (1-4) hours. Also cement particles start to form initial layer of hydration
product. Bleeding and sedimentation appear in this stage. Third: Next heat evolution is on
account of dissolution of weak gel which formed first on surfaces of C3S crystals so water will
reach these surfaces and starts to form new gel. This stage last about 6 hours. Forth: in this stage
cement begin to harden and gain strength.
False and flash Setting
False setting is the rapid development of rigidity in freshly mixed Portland cement paste,
mortar, or concrete occurs after few minutes of mixing cement with water without the
generation of much heat. It can be controlled by re-mixing without additional water or strength
loss. Causes of false setting are:
1. Drying of gypsum: when gypsum (CaSO4.2H2O) gridding with hot clinker it loss 75% of
its water and formed (CaSO4.1/2 H2O) and if temperature of clinker increase gypsum loss
all water in its composition and formed CaSO4. When water add to cement CaSO4 react
causes rapid setting.
2. Bad storage: alkalis in cement react with carbon dioxide forming Alkali carbonate which
reacts with calcium hydroxide forming CaCO3 causing setting of cement paste.
3. Activate effective C3A exposed to humidity: during bad storage water adhere on cement
particles surface and during mixing these active surface combined rapidly with water
within minutes.
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Flash setting is a rapid development of rigidity in freshly mixed Portland – cement paste,
mortar, or concrete. Further mixing can’t dispel this rigidity, and a large amount of heat is
produced in the process. It happened Due to rapid reaction of aluminates – when insufficient
sulfate present so to prevent It A small amount of gypsum (CaSO4·2H2O) gridding with the
cement clinker retards the hydration reaction of tricalcium aluminate so that the calcium
silicates can set firstly.
Soundness of Cement
It refers to the ability of a hardened cement paste to retain its volume after setting without
delayed destructive expansion. This destructive expansion is caused by excessive amounts of
free lime (CaO), (SO3) or magnesia (MgO).
The hydrating speed of the over burnt CaO or MgO is slow, CaO or MgO begins to hydrate
after cement hardening and causes the hardened cement to expand and crack.
When added too much, gypsum continues to react with calcium aluminate hydrate to form
calcium sulfoaluminate hydrate, whose volume increase 1.5 times as big as gypsum and causes
the hardened cement paste to crack. At this time, sulfoaluminate hydrate is called cement
bacillus.
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